The Cardassian design of this weapon was designed specifically for planetary assault and thus was incapable of being used against starships. The plasma cannon was slung underneath the ship, where it launched a super-dense projectile at a planet's surface. Once the planet was struck, a significant amount of people were killed by the weapon's blast. (TNGvideo game: Armada II)

Presumably, starships were capable of avoiding this version of the weapon which was why it was not used against starships.